Project Summary The forces driving the current obesity epidemic remain a matter of debate, but obesity has largely been assumed to result from increased caloric intake. However, diets high in fat result in increased adiposity even in the absence of increased caloric intake. These findings suggest that dietary fats play an important role in increased WAT mass in obesity. While the amount of fat in the diet has increased during the obesity epidemic, the types of fats in the diet that have changed even more drastically. However, our understanding of how specific fatty acids affect the onset of obesity, fat distribution and metabolic disease is limited. Since mature adipocytes are post-mitotic, they are generated from the proliferation and differentiation of adipocyte precursors (APs). We have recently identified the adipocyte cellular lineage in vivo, and we have developed assays to directly interrogate the cellular and molecular mechanisms that control AP contribution to WAT mass in physiologically relevant contexts in vivo. We have since characterized when adipogenesis occurs in response to high fat diet (HFD) feeding and have established that HFD-induced adipogenesis occurs in specifically in visceral WAT of male mice. In addition, we found that HFD-induced adipocyte formation requires Akt2 in the adipocyte cellular lineage, but Akt2 null mice establish WAT mass and adipocyte number normally during development. These findings indicate that the molecular regulation of adipogenesis in obesity is distinct from developmental adipogenesis. To establish how HFD induces adipogenesis in obesity, we have determined the adipogenic response to HFDs composed of different fat sources. We found only diets high in monounsaturated fats induce adipocyte hyperplasia and that these adipogenic fat sources affect fat distribution in obesity. As oleic acid is the major dietary monounsaturated fatty acid, and we have shown that oleic acid can function as a signaling molecule to induce adipogenesis directly on APs, we hypothesize that dietary oleic acid acts as a signaling molecule that drives adipocyte hyperplasia in obesity in mice and humans. Preliminary data indicates that oleic acid simulated adipogenesis in primary APs is dependent on Akt2 and the fatty acid receptor GPR120. Here we will characterize the dietary components that affect adipocyte hyperplasia, determine the metabolic consequences of oleic acid induced adipogenesis and define the molecular mechanisms that control adipocyte hyperplasia in both mouse and human models. These studies will improve our understanding of how modern diets may have contributed to the obesity epidemic and metabolic disease, potentially leading to new dietary recommendations and novel therapeutic strategies for sustained weight loss and improved health outcomes.